In European patent application EP-A 0 357 513 there is described a hydrocarbon dispenser comprising means for controlling the gas content in the hydrocarbon being dispensed. As is known, a hydrocarbon dispenser comprises not only a shutoff valve in the dispenser nozzle but also a main valve upstream of the dispenser nozzle hose, accordingly between the dispensing pump and the dispenser nozzle. In the cited application, there is described a system allowing the closure of the main valve to be brought about automatically when the gas content in the hydrocarbon is above a predetermined percentage. To this end a system either allows the control chamber of the main valve to be at atmospheric pressure when the gas content is normal, thus allowing the open position of the main valve to be maintained, or shuts off the control chamber, which effects closure of the main valve through increasing the pressure in the chamber, even if the valve of the dispenser nozzle is kept open, i.e. even if the dispensing pump continues to operate.
The problems which can arise in relation to the main valve will be better understood with reference to the accompanying FIGS. 1a and 1b.
The valve of the European application referred to above is shown in FIG. 1a, in its open position. The valve comprises a piston 22 sliding in a control chamber 38. The control chamber is normally under atmospheric pressure through the pipe 36. The piston 22 can cooperate with a seat 19 which surrounds the outlet pipe 18 leading to the dispenser nozzle. Also shown is the pipe 16 through which the hydrocarbon fed by the pump is received. The piston 22 tends to be applied against its seat 19 by a return spring 44 fitted in the chamber 38. When the gas content of the hydrocarbon is normal, the chamber 38 is at atmospheric pressure. When the dispenser is operating, the high pressure which obtains in the pipe 16 acts on the face 22a of the piston 22, causing the main valve to open. When the gas content exceeds a predetermined value, the pipe 36 is closed and, because of the opening 40 formed through the piston 22, the high pressure applied through the pipe 16 is established in the control chamber 38 also, which causes the main valve to close under the action of the spring, the piston 22 coming into contact with its seat 19.
In FIG. 1b there is shown the main valve in the closed position, i.e. with the piston 22 on its seat 19. The diameter of the piston 22 is denoted D and the diameter of the seat 19d, which is substantially that of the pipe 18.
In normal operation, when atmospheric pressure PA obtains in the chamber 38, and the high pressure HP is applied via the duct 16, the equilibrium of the piston 22 can be expressed as follows: EQU (HP-PA).multidot..pi.(D.sup.2 -d.sup.2)+(LP-PA).multidot..pi.d.sup.2 =F
where F is the compressive force exerted by the spring 44, LP is the low pressure in the pipe 18 following closure of the main valve, and HP is the high pressure in the pipe 16.
It will be seen that, when the main valve re-opens, the pressure LP contributes to this opening so long as it is above atmospheric pressure PA. On the contrary, if the pressure LP is lower than atmospheric pressure, it creates a partial vacuum which opposes opening of the main valve when the pressure HP is applied. It is easy to see that, depending on the amount of the under-pressure in the pipe 18 and depending on the value assumed by the high pressure HP, a situation can arise in which it will not be possible to get the main valve to open by the application of the high pressure in the duct 16.
Two main cases can be seen of situations which can create an under-pressure in the pipe 18.
The first situation is that in which the hydrocarbon dispenser is not operating, i.e. the valve of the dispenser nozzle is closed. Under these circumstances, a vehicle running over the hose of the dispenser nozzle creates an excess hydrocarbon pressure in the hose. This excess pressure introduced into the hose and thus into the pipe 18 is sufficient to cause the piston 22 to lift. This results in the transfer of a fraction of the hydrocarbon in the duct 18 to the duct 16, in correspondence with the squashing of the hose. As soon as enough liquid has been transferred to bring down the pressure in the duct 16, the piston can return to its seat under the action of the spring. When the wheel moves off the hose, this tends to regain its initial volume, creating an underpressure as a result of the lack of liquid relative to the prior situation. The partial vacuum in the hose of the nozzle is made possible by its stiffness. In some cases it has been possible to observe an under-pressure of 700 millibars with some types of hydrocarbons.
Another situation which can involve the creation of an under-pressure in the pipe 18 is as follows. When an abnormal gas content is detected, the main valve 22 closes abruptly under the action of the rapid increase of pressure in the control chamber 38. However the inertia of the column of liquid between the valve 22 and the dispenser nozzle at the end of the hose has to be taken into account. This column of liquid is moving at a speed of several meters per second during the supply of hydrocarbon. The abrupt closure of the piston 22 does not allow all of the flow in the region of the nozzle to stop immediately, so that an under-pressure occurs which is established in the hose and hence in the outlet pipe 18.
In order to overcome this problem it is an object of the present invention to provide a valve with a control chamber with means to control the speed of complete closing during the application of pressure in the control chamber.